Xenobiotic chemicals can be metabolized by organs and tissues other than the liver. Additionally, the prostaglandin endoperoxide synthetase (PES) system is found in a number of organs and is not dependent on cytochrome P-450. The metabolism of polycyclic aromatic hydrocarbons, aromatic amines, and other chemicals to mutagens in Salmonella tester strains is being studied using PES and, also, microsomal preparations from lungs, testes, and prostate, in addition to liver. Only the dihydrodiols of polycyclic aromatic hydrocarbons which allow formation of bay region diolepoxides were mutagenic in the presence of PES. The majority of aromatic amines studies were also mutagenic, but a number of them were weak or nonmutagenic for reasons not evident. N-Nitrosamines were not active in this system. Testes and prostate microsomal mixed function oxidases are inducible and homogenates from these organs can metabolize benzo(a)pyrene to a mutagen.